Electric double-layer capacitor

ABSTRACT

The invention provides an electric double-layer capacitor sealed up by a flexible casing material that ensures a release of pressure upon an increasing internal pressure. A pressure release valve ( 1 ) is attached to the flexible casing material. The pressure release valve ( 1 ) comprises a collar airtightly joined to the flexible casing material and a cylindrical portion ( 3 ) connected to the collar, extending out of a capacitor enclosure. The cylindrical portion ( 3 ) comprises an end part ( 5 ) having a self-closing passage ( 6 ) that is opening to the outside only upon a pressure release and is closed up in a normal state, and a channel ( 4 ) in communication with the interior of the capacitor enclosure.

BACKGROUND OF THE INVENTION

The present invention relates generally to an electric double-layercapacitor, and more particularly to an electric double-layer capacitorhaving a restorable pressure release valve that is sealed up by aflexible casing material.

Making use of an electric double-layer capacity formed at an interfacebetween a polarizable electrode and an electrolyte, an electricdouble-layer capacitor has the feature of being much larger in capacitythan that comprising electrodes with a dielectric material interposedbetween them, and is now under development as promising power storagemeans.

Electric double-layer capacitors known so far in the art are broken downinto two types, one type using an aqueous solvent electrolyte andanother relying upon a non-aqueous solvent electrolyte. The aqueoussolvent type is low in applied voltage due to the electrolysis voltageof water and, in contrast thereto, the non-aqueous solvent type has agreat feature of being operated at higher voltage than the aqueoussolvent type.

Referring to the electric energy stored in the capacitor, storageefficiency of electric energy increases with an increasing appliedvoltage, as can be seen from the unit ½ CV² for electric energy. Thus,the electric double-layer capacitor of the non-aqueous solventelectrolyte type can be well fit for energy storage such as powerstorage.

Problems with the electric double-layer capacitor of the non-aqueoussolvent electrolyte are that as atmospheric moisture enters theelectrolyte, the electrolysis voltage of the electrolyte decreases orthe non-aqueous solvent electrolyte decomposes. For the electricdouble-layer capacitor using a non-aqueous solvent electrolyte,therefore, a capacitor enclosure is sealed up to prevent entrance ofatmospheric moisture therein.

When the electric double-layer capacitor is used as a condenser, it isof vital importance that the output or energy density of the capacitoris high; the output and energy per volume or weight are large. For thisreason, an electric double-layer capacitor enclosed in a flexible casingmaterial obtained by lamination of a synthetic resin film of small massand an aluminum foil or the like rather than in a metallic can materialwould be preferable.

The electric double-layer capacitor has an extended charge-dischargecycle life and can be used over an extended period of time, becausedegradation and deterioration of electrodes are limited due to theabsence of such chemical reactions of materials as found in secondarybatteries. However, the demerit is that internal pressure rises due tothe decomposition of the materials used such as electrolyte and thedesorption of moisture adsorbed in the materials during long-term use.

Generally speaking, as internal pressure rises in non-aqueouselectrolyte secondary batteries such as lithium ion batteries due to gasgeneration, abnormal electrochemical reactions proceed to the pointwhere the batteries deteriorate irreparably. For these batteries,therefore, irreparable pressure release valves comprising thin portionsthat are cleaved or broken are used.

On the other hand, the electric double-layer capacitor has the featureof continuing to work even after internal pressure is released off byoperation of the pressure release valve, on condition that theelectrolyte is kept in a given amount. For this electric double-layercapacitor, therefore, it is very significant means to rely on arestorable pressure release valve. For such a restorable pressurerelease valve, a number of arrangements making use of spring resiliencyor the like have been generally known in the art. For an electricdouble-layer capacitor using a flexible casing material such as asynthetic resin film, however, there is still no small-size restorablepressure release valve that is easily joinable to the flexible casingmaterial.

The inventors have already filed Japanese Patent Application No.2001-80180 to come up with an electric double-layer capacitor comprisinga pressure release valve that uses a flexible casing material. Dependingon the states of components of the pressure release valve, however,there are inconveniences such as deposition of emissions resulting fromthe supporting electrolyte materials in the electrolyte onto the outsideof the pressure release valve, and a rupture of the flexible casingmaterial.

The inventors have also filed Japanese Patent Application No.2001-294179 to propose an electric double-layer capacitor to which apressure release valve having a self-closing passage through it isattached.

This pressure release valve is typically shown in FIG. 13. As shown, aconventional pressure release valve 21 comprises a member of rubberelasticity and a flexible casing material joined directly thereto, andhas the feature of being capable of attachment with no additionaldevice.

A problem with this pressure release valve 21 that comprises anelastomer having a self-closing passage and is attached to a flexiblecasing material 22 is that internal pressure is often not released at aprecisely preset pressure due to deformation of the elastomer and theflexible casing material upon an increasing pressure.

Another problem is that upon actuation of the pressure release valve,the pressure release valve does not effectively work due toprecipitation of supporting electrolytic materials in the electrolyte.

A primary object of the invention is to provide an electric double-layercapacitor that uses a non-aqueous solvent electrolyte and is sealed upby a flexible casing material, which comprises a pressure release valvethat is actuated precisely in response to an abnormal pressure rise torelease off internal pressure, so that the electric double-layercapacitor can maintain its own performance over an extended period oftime.

SUMMARY OF THE INVENTION

Thus, the present invention provides an electric double-layer capacitorsealed up by a flexible casing material, wherein:

-   -   a pressure release valve is attached to said flexible casing        material,    -   said pressure release valve comprises a collar air-tightly        joined to the flexible casing material and a cylindrical portion        connected to the collar, extending out of a capacitor enclosure,        and    -   said cylindrical portion comprises an end part having a        self-closing passage that is opening to the outside only upon a        pressure release and is closed up in a normal state and a        channel in communication with an interior of the capacitor        enclosure.

Preferably, said collar is provided thereon with a layer of materialthat is easily joinable to an associated joining surface of the flexiblecasing material.

Preferably, said end part having a self-closing passage is of a conicalor truncated cone shape that diminishes in diameter toward a pointedend.

Preferably, said end part having a self-closing passage is provided witha member that limits outward extension of said end part.

Preferably, said end portion having a self-closing passage is formed ofan olefinic synthetic rubber.

Preferably, said olefinic synthetic rubber is a polymer blend comprisinga polypropylene and an ethylene-propylene-diene copolymer.

Preferably, said channel is provided with a porous member having acombined liquid repellency and gas transmission.

Preferably, the porous member comprises a fluororesin.

Preferably, said pressure release valve is connected with an emissiondischarge conduit.

The present invention also provides a capacitor pack having a pluralityof electric double-layer capacitors received in capacitor enclosures,wherein each electric double-layer capacitor is provided with saidpressure release valve, and an emission discharge conduit is connectedto said pressure release valve in such a way that an end of saidemission discharge conduit is located externally of said capacitorenclosure.

BEST MODE OF CARRYING OUT THE INVENTION

With the electric double-layer capacitor sealed up by a flexible casingmaterial according to the invention, it has been found that by providinga self-closing passage and a channel in communication with the interiorof the capacitor enclosure through a cylindrical elastomer portion ofrubber elasticity extending externally from the wall surface of theflexible casing material, it is possible to provide a pressure releasevalve that ensures a release of internal pressure without being affectedby deformation or the like due to a pressure difference even when theinternal pressure of the electric double-layer capacitor increases and,upon restoration, enables the interior of the capacitor enclosure to bekept airtight from outside.

By locating a porous member comprising fluororesin on the channel, it isalso possible to provide an electric double-layer capacitor that cantransmit gases upon an increasing pressure and, at the same time, canprevent leakage of a non-aqueous electrolyte and precipitation with thepressure release valve of a supporting electrolyte, etc. from theelectrolyte.

In other words, it has been found that although a polar material used asthe solvent for a non-aqueous electrolyte such as ethylene carbonate orpropylene carbonate has liquid repellency with respect to the porousfilm member composed of fluororesin or the like, the aforesaid porousfilm member having a given porosity can transmits gasses alone whileshielding off the non-aqueous electrolyte.

An arrangement wherein an elastomer of rubber elasticity having aself-closing passage is positioned within a flexible casing material ischaracterized in that there are no protrusions, etc. on the outersurface of an enclosure. A problem with such an arrangement, however, isthat difficulty is encountered in a precise release at a given pressureof internal pressure through the self-closing passage, because theelastomer is pressurized from outside by an increasing pressure withinthe enclosure. According to the invention, however, it is possible toprovide an electric double-layer capacitor having a pressure releasevalve and received in an enclosure comprising a flexible casingmaterial, in which by providing a self-closing passage and a channel incommunication therewith on the outside of the casing material, so thatthe pressure release valve is protected against poor operation due todeformation upon an increasing pressure.

The present invention is now explained with reference to theaccompanying drawings.

FIGS. 1(A) and 1(B) are a plan view and a sectional view of oneembodiment of the pressure release valve attached to a flexible casingmaterial, which is used with the electric double-layer capacitoraccording to the invention.

A pressure release valve 1 comprises a collar 2 to be joined to aflexible casing material for the electric double-layer capacitor and acylindrical portion 3 that extends out of the flexible casing materialwhen the collar 2 is attached to an associated joining surface of theflexible casing material. The cylindrical portion 3 has therein achannel 4 in communication with the interior of a capacitor enclosure,and is provided through an end part 5 with a self-closing passage 6 thatis usually airtightly closed up by rubber elasticity, and is operable tocommunicate with the interior of the capacitor enclosure at anincreasing pressure.

As shown in FIG. 1(B), the cylindrical portion is of a truncated coneshape wherein the end part 5 diminishes in diameter toward its pointedend. While the cylindrical portion may be of the same diameter, it isunderstood that the above tapering configuration ensures smootherpressure release operation. In FIGS. 1A and 1B, the self-closing passageis in an open state.

Although the joining surface of the collar 2 to the flexible casingmaterial may be either a surface on which the cylindrical portion 3 islocated or its opposite surface, it is preferable that the surface ofthe collar with the cylindrical portion 3 located thereon is joined toan easily joining surface of the flexible casing material, and thecollar 2 is located within the capacitor enclosure formed of theflexible casing material.

While the self-closing passage may be formed of a hole of circular orrectangular shape in axially vertical section, it is preferable that thecylindrical portion 3 is holed, grooved or otherwise processed by meansof a pointed tool without wasting material.

The self-closing passage could be provided after attachment of thepressure release valve to the electric double-layer capacitor. If theself-closing passage is provided after assembling of the electricdouble-layer capacitor, leakage testing for the sealed site of theelectric double-layer capacitor or the joint of the pressure releasevalve to the flexible casing material can then be easily performed.

FIGS. 2(A) and 2(B) are illustrative of another embodiment of thepressure release valve in the electric double-layer capacitor of theinvention.

A pressure release valve 1 depicted in FIG. 2(A) comprises a collar 2 tobe joined to a flexible casing material and a cylindrical portion 3 thatextends out of the flexible casing material when the collar 2 isattached to the inside surface of the flexible casing material.

The collar 2 is provided with a joining layer 7 formed of a materialthat is different from a rubber elastomer forming part of the pressurerelease valve 1 and joins easily to the joining surface of the flexiblecasing material. The provision of the joining layer 7 ensures that thepressure release valve can join easily to the flexible casing material,and the formation of the layer of different material on the collarensures that the rigidity of the collar increases. It is thus possibleto obtain a pressure release valve that has greater strength, and isless deformable so that it can work under more precise pressure.

Referring to FIG. 2(B), a covering layer 8 is applied to the surface ofa cylindrical portion 3 and its end part, leading to the joining layer 7formed on the collar of the pressure release valve.

The application of the covering layer 8 makes the rigidity of the rubberelastomer forming part of the pressure release valve greater. It is thuspossible to obtain a pressure release valve of greater strength.

In FIGS. 2(A) and 2(B) alike, the self-closing passage remains open.

FIGS. 3(A), 3(B) and 3(C) are illustrative of yet another embodiment ofthe pressure release valve in the electric double-layer capacitor of theinvention.

FIG. 3(A) shows a step 9 provided at the cylindrical portion 3 of thepressure release valve depicted in FIG. 1, wherein a end part 5 islocated on the step 9. FIG. 3(B) shows a step 9 at the cylindricalportion 3 of the pressure release valve depicted in FIG. 2(A), and FIG.3(C) shows a step 9 at the cylindrical portion 3 of the pressure releasevalve depicted in FIG. 2(B).

By the provision of the step it is possible to improve the degree offreedom in determining the size of the cylindrical portion of thepressure release valve and designing its end part.

Throughout FIGS. 3(A), 3(B) and 3(C), a self-closing passage 6 is in anopen state.

FIGS. 4(A) and 4(B) are a plan view and a sectional view of a furtherembodiment of the pressure release valve in the electric double-layercapacitor of the invention.

A pressure release valve 1 comprises a collar 2 to be joined to aflexible casing material and a cylindrical portion 3 that extends out ofthe flexible casing material when the collar 2 is attached to the insidesurface of the flexible casing material.

The cylindrical portion 3 is provided with a step 9 and an end part 5that is smaller in diameter than the cylindrical portion 3. At a site ofthe step 9 facing away from a self-closing passage 6, there is provideda pressure adjuster 10. The pressure adjuster 10 is operable to keep theself-closing passage 6 from becoming wide due to a pressure differencebetween inside and outside a capacitor enclosure, and prevent thepressure release valve from deformation due to a rise in the internalpressure, which may otherwise cause the pressure release valve to workat a pressure lower than a predetermined pressure.

This arrangement allows the pressure for releasing the internal pressureto be greater than that in the absence of the pressure adjuster. Thus,the provision of the pressure adjuster permits a pressure release valveof the same configuration to be operable at a higher pressure.

Preferably, the pressure adjuster 10 should be located at a position ofthe end part of the cylindrical portion that faces away from theself-closing passage. For the pressure adjuster 10, for instance, a ringmember or a V-grooved member could be used provided that they can limitthe valve-opening operation of the self-closing passage. Preferably, thecylindrical portion to which the pressure adjuster is to be attachedshould be provided with a groove or step that conforms to the pressureadjuster.

The pressure adjuster 10 could be formed of synthetic resin, metalmaterial or the like, and the valve-opening pressure of the self-closingpassage could be adjusted in dependence on the thickness or aperturediameter of the pressure adjuster.

In FIGS. 4(A) and 4(B) alike, the self-closing passage is in an openstate.

FIGS. 5(A) and 5(B) are illustrative of a further embodiment of thepressure release valve in the electric double-layer capacitor of theinvention.

In a pressure release valve depicted in FIG. 5(A), a joining layer 7 isformed on the collar 2 of the pressure release valve illustrated in FIG.3(B), and a pressure adjuster 10 is fitted to a step 9 at a cylindricalportion 3. In a pressure release valve depicted in FIG. 5(B), a coveringlayer 8 is applied to the surface of the cylindrical portion 3 and endpart 5 of the pressure release valve illustrated in FIG. 3(C), and apressure adjuster 10 is fitted to the step 9 at the cylindrical portion3. With such an arrangement, it is possible to achieve a pressurerelease valve that has greater rigidity and higher valve-openingpressure.

In both FIGS. 5(A) and 5(B), the self-closing passage is in an openstate.

The pressure release valve of the invention could be configured into anydesired shape in a site-dependent manner; for instance, it could beconfigured into a cylindrical or rectangular shape.

The rubber elastomer material used for the electric double-layercapacitor of the invention should ensure that a self-closing passage canbe created by rubber elasticity, and includes, for instance, olefinicsynthetic rubbers such as ethylene propylene copolymers (EPT) andethylene-propylene-diene copolymers (EPDM), silicone rubbers, andfluororubbers, all stable with respect to a non-aqueous electrolyte.

Preferably, the joining layer to be formed at the collar should be madeup of a material that can join easily to the flexible casing material;for instance, polyethylene, and polypropylene having good fusionbondability with respect to a polyethylene or polypropylene layer usedfor lamination to an aluminum foil are usable as the joining layer thatcan join easily to the flexible casing material.

That joining layer could have been modified on its surface to improveits joining capability to the joining surface of the flexible casingmaterial.

If the pressure release valve is molded using a material of rubberelasticity with a polyethylene or polypropylene thin film for thejoining layer placed in a mold, it is then possible to obtain thejoining layer by means of integral molding.

For the rubber elastomer material, just only the olefinic rubber such asEPDM but also a thermoplastic elastomer comprising polypropylene andethylene-polypropylene-diene copolymer (EPDM) could be used. Thethermoplastic elastomer comprising EPDM blended with polypropyleneimproves joining capability upon fusion bonding onto the surface of athermoplastic film such as a polyethylene or polypropylene film, and somakes it possible to obtain an electric double-layer capacitor that canshow stable performance over an extended period of time.

The amount of polypropylene in such a thermoplastic elastomer should bein the range of preferably 5 to 40 parts by weight, more preferably 15to 25 parts by weight per 100 parts by weight of EPDM. Less than 5 partsby weight of polypropylene are less effective on improvements in thejoining capability of EPDM, and more than 40 parts by weight have anadverse influence on rubber elasticity or the like, causing sealingcapability or the like to become worse.

If the joining surface of the rubber elastomer is treated as bypolishing to form minute asperities thereon, stronger joining force isthen obtained.

FIG. 6 is a partly taken-away perspective view of one embodiment of theelectric double-layer capacitor of the invention.

An electric double-layer capacitor 23 uses a non-aqueous solventelectrolyte. A capacitor element 24 is sealed up by a flexible casingmaterial 22, and a collar of a pressure release valve 1 made up of arubber elastomer material is fusion bonded to the inside surface of theflexible casing material 22 in such a way as to extend out of theflexible casing material 22.

The pressure release valve 1 is mounted on top of the electricaldouble-layer capacitor in such a way as to be flush therewith, so thattwo or more electric double-layer capacitors can be placed one uponanother without offering any problem.

Because the material having rubber elasticity is used for the pressurerelease valve 1, the self-closing passage is shut off when the pressuredifference between inside and outside the electric double-layercapacitor becomes smaller than a predetermined one.

An electric double-layer capacitor can be reused after a release ofpressure from within by means of a restorable pressure release valve. Ina normal state, entrance of moisture or the like is preventable byrelatively small tight force rather than large tight pressure, and sothe electric double-layer capacitor can work well. Thus, the pressurerelease valve of the invention is best fit for such an electricdouble-layer capacitor.

In the invention, a pressure release valve previously provided with agiven self-closing passage could be used; however, it is acceptable touse a separately prepared pressure release valve with no self-closingpassage. In this case, an electric double-layer capacitor is prepared byattaching that pressure release valve to a flexible casing material, andthen housing an electric double-layer capacitor element within acapacitor enclosure to seal up the capacitor assembly with the flexiblecasing material. Then, leakage testing is carried out for the sealedsite of the electrical double-layer capacitor, the joint of the pressurerelease valve to the electric double-layer capacitor, etc. Finally, theself-closing passage is formed by providing an opening through thepressure release valve by means of a tool having a pointed edge.

FIGS. 7(A), 7(B) and 7(C) are illustrative of a further embodiment ofthe pressure release valve in the electric double-layer capacitor of theinvention, wherein a porous member having liquid repellency and gastransmission is located in place.

Each of pressure release valves 1 depicted in FIGS. 7(A), 7(B) and 7(C)comprises a pressure release valve 1 depicted in FIG. 1, FIG. 2(A), andFIG. 2(B) and a porous member 11 attached thereto.

The pressure release valve 1 comprises a collar to be joined to aflexible casing material of an electric double-layer capacitor and acylindrical portion 3 that extends out of the flexible casing materialwhen the collar 2 is attached to the joining surface of the flexiblecasing material. The cylindrical portion 3 has a channel 4 that is incommunication with the interior of a capacitor enclosure. An end part 5of the cylindrical portion 3 has a self-closing passage 6 that openswhen the internal pressure of the capacitor enclosure increases, andremains shut off by rubber elasticity in a normal state to keep airtightness. Referring to FIG. 7(B), a joining layer is formed at thecollar 2 to increase rigidity and adhesion to the flexible casingmaterial, and referring to FIG. 7(C), a covering layer 8 is applied onthe surface of the end part 5, leading to the joining layer 7.

The porous member 11 having a combined liquid repellency and gastransmission is provided in such a way as to cover the channel 4.

Having open cells to ensure air permeation, the porous member 11 isformed of a fluororesin that is made porous by stretching, foaming,extraction or the like. The fluororesin, for instance, includespolytetrafluoroethylene, tetrafluoroethylene-hexafluoropropylenecopolymers, polyvinyl fluoride, and polyvinylidene fluoride. Preferenceis given to a porous film obtained by cleaving polytetrafluoroethyleneby stretching to form minute pores therein.

The air permeability and liquid repellency of the porous film depends onthe material of the fluororesin used as well as the porosity, thickness,pore diameter, etc. of the porous film, and as porosity increases,liquid repellency would tend to become low. On the other hand, lowerporosity would detract from air permeability. Smaller pore diameterwould cause liquid repellency to increase but air permeability todecrease. Because thickness is a factor of causing strength drops andproducing pinholes, thickness of appropriate magnitude is preferablymade from a thickness range that is not detrimental to air permeability.

Porosity, pore diameter and so on are determined in dependence on theproperties of the liquid contained in a flexible enclosure with whichthe pressure release valve of the invention is used; however, it isdesired to use a porosity of 1 to 90%, preferably 3 to 50%, a porediameter of 0.02 to 3.0 μm, preferably 0.02 to 0.2 μm, and a thicknessof 20 to 1,000 μm, preferably 100 to 1,000 μm. Alternatively, thefluororesin having improved liquid repellency could be integrated withan unwoven or woven fabric of chemical-resistant polypropylene or otherresin.

Although the porous film of the fluororesin suitable as a member ofliquid repellency and air permeability is hardly joined to anapplication member by means of fusion bonding, it is understood that ifthat porous film is heated in close contact with the pressure releasevalve to be joined, both can then be closely joined together because thematerial of rubber elasticity forming the pressure release valve meltsto penetrate in minute cells throughout the porous film. When thematerial of rubber elasticity is molded into a pressure release valve,the porous film could be placed in a mold for integral moldingtherewith.

The pressure release valve with a liquid repellant member attached to itensures that an electrolyte is prevented from reaching the self-closingpassage, and in a normal state, the self-closing passage can be reliablysealed up and the electric double-layer capacitor can be depressurizedat a given pressure.

FIGS. 8(A) to 8(E) are illustrative of a further embodiment of thepressure release valve in the electric double-layer capacitor of theinvention, wherein a porous member of liquid repellency and gastransmission is positioned in place.

A pressure release valve 1 depicted in FIGS. 8(A), 8(B), 8(C), 8(D), and8(E) comprises the pressure release valve 1 depicted in FIGS. 3(A),3(B), 3(C), 5(A), and 5(B) with a porous member 11 attached to it.

In FIG. 8(A), a cylindrical portion 3 of the pressure release valve isprovided with a step 9 having an end part 5, and in FIG. 8(B), a collar2 of the pressure release valve is provided thereon with a joining layerso as to improve rigidity and adhesion to a flexible casing material. InFIG. 8(C), a covering material 8 is applied onto the surface of an endpart 5, leading to a joining layer 7, for the purpose of enhancingrigidity. In FIG. 8(D), the step 9 of the pressure release valvedepicted in FIG. 8(B) is provided with a pressure adjuster 10, and inFIG. 8(E), a pressure adjuster 10 is placed at the step 9 depicted inFIG. 8(C). With each of those arrangements, it is possible to provide apressure release valve of high rigidity and an elevated valve-openingpressure.

A porous material 11 having a combined liquid repellency and gastransmission is positioned in such a way as to cover a channel 4 throughthe collar.

The arrangements of FIGS. 8(A)-8(E) have the same advantages as do thoseof FIGS. 7(A)-7(C), ensuring that the electrical double-layer capacitorcan be sealed up, and the pressure can be released out of it upon anabnormal pressure rise.

FIG. 9 is illustrative of another embodiment of the electricdouble-layer capacitor of the invention, wherein the pressure releasevalve is provided with an emission discharge conduit.

When the pressure release valve is actuated in response to an increasein the internal pressure of the inventive electric double-layercapacitor employing a non-aqueous electrolyte, mists of the non-aqueouselectrolyte are often discharged along with internal moisture, etc.,resulting in deposition to the flexible casing material that surroundsthe pressure release valve, or the conductive terminal. However, if anemission discharge conduit 12 is attached to the pressure release valve1, emissions can then be discharged out of any position spaced away froman electric double-layer capacitor 23, averting adverse influencesthereon. For the emission discharge conduit, a tube formed ofpolyethylene or other synthetic resin is attached onto the cylindricalportion of the pressure release valve 1 or onto the pressure adjuster.

FIG. 10 is a plan view of one embodiment of a capacitor pack thatreceives a plurality of electric double-layer capacitors, wherein a lidmember is put off.

One single electric double-layer capacitor of the invention is barely 2to 3 V higher in rated voltage than that using an aqueous electrolyte,and does not well service as an accumulator. Therefore, a number ofelectric double-layer capacitors are connected in series and parallelfor use in the form of a capacitor pack 13.

In the capacitor pack 13 shown, a number of electric double-layercapacitors 23 are received in a housing 14, with conductive connectorterminals 15 connected in series via inter-terminal conductors 16.

A pressure release valve 1 in each electric double-layer capacitor 23 isconnected with a conduit 12 for discharge of emissions occurring duringits operation. The emission discharge conduit 12 is joined to removalmeans 17 positioned externally of the housing 14, where componentscontained in the non-aqueous electrolyte are removed by adsorption orother means for spewing to the atmosphere. The removal means 17 could befilled with an adsorbent such as active charcoal. The thus assembledcapacitor pack can run stably over an extended period of time, becausemists of the non-aqueous electrolyte are not fed back to the housing,and there is no poor insulation due to deposition of mists.

The present invention is now explained with reference to examples.

EXAMPLE 1

A truncated cone form of ethylene-propylene-diene copolymer pressurerelease valve was provided, which comprised a collar of 1.5 mm inthickness and 15 mm in diameter, a step having an outside diameter of 4mm, an inside diameter of 1 mm and a height of 2 mm from the collar andan end portion having an outside diameter of 3.5 mm at the step and anend part of 1 mm in diameter, as shown in section in FIG. 11. Thepressure release valve was then fusion bonded to a flexible casingmaterial for an electric double-layer capacitor, which had a three-layerstructure of propylene film/aluminum foil/polyester film, whereuponthree sides of the flexible casing material were sealed up with apressure testing tube attached thereto, thereby making an airtightenclosure.

After airtight testing was carried out for the sealed skite of theairtight enclosure and the portion of the pressure release valve fusionbonded thereto, a pointed needle of 0.5 mm in diameter was stuck throughthe end part of the pressure release valve to form a self-closingpassage.

Compressed air was injected into the thus obtained 10 enclosures untilactuation of the pressure release valves. By measurement, the pressurerelease valves were was found to be actuated at a pressure of 0.022 MPa(gauge pressure).

EXAMPLE 2

Compressed air was injected in 10 enclosures prepared as in Example 1with the exception that a pressure release valve having a self-closingpassage of 4 mm in length was used. The pressure release valves werefound to be actuated at a pressure of 0.032 MPa (gauge pressure).

EXAMPLE 3

Compressed air was injected in 10 enclosures prepared as in Example 1with the exception that a pressure release valve having a collarintegral with a 40-μm thick polypropylene film was used. The pressurerelease valves were found to be actuated at a pressure of 0.042 MPa(gauge pressure).

EXAMPLE 4

Compressed air was injected in 10 enclosures prepared as in Example 1with the exception of attachment of a pressure adjuster of 3.0 mm ininside diameter and 1.0 mm in thickness to an outer site of thecylindrical portion facing away from the self-closing passage. Thepressure release valves were found to be actuated at a pressure of 0.072MPa (gauge pressure).

EXAMPLE 5

Thirty (30) electric double-layer capacitors were provided, each havinga pressure release valve of Example 1 attached to a flexible casingmaterial. Each capacitor was subjected to accelerated testing ofrepeating a charge-discharge cycle every 10 hours on a current of 2.7 Vand 10 A. Throughout the capacitors, the pressure release valves werenormally operated with an increasing internal pressure until 1,000hours. In four capacitors, however, the casing materials broken due toinactivation of the pressure release valves by deposition of solidemissions precipitated from the electrolytes.

EXAMPLE 6

A truncated cone form of ethylene-propylene-diene copolymer pressurerelease valve was provided, which comprised a collar of 1.5 mm inthickness and 15 mm in diameter, a step having an outside diameter of 4mm, an inside diameter of 1 mm and a height of 2 mm from the collar andan end portion having an outside diameter of 3.5 mm at the step and anend part of 1 mm in diameter, as shown in section in FIG. 12. A porouspolytetrafluoroethylene film having a thickness of 300 μm and a porosityof 80% was provided in such a way as to cover a channel through thecollar. The thus obtained pressure release valve was fusion bonded to aflexible casing material having a three-layer structure of propylenefilm/aluminum foil/polyester film. After this, 30 electric double-layercapacitors were prepared and subjected to charge-discharge cycle testingas in Example 5. Even after the lapse of 1,000 hours, the pressurerelease valves were kept airtight, with no breaking of the casingmaterials.

EXAMPLE 7

For the sake of comparison in terms of joining strength to a flexiblethree-layer casing material of a three-layer structure of propylenefilm/aluminum foil/polyester film, there were provided a pressurerelease valve made up of an ethylene-propylene-diene copolymer that didnot contain any polypropylene, and a pressure release valve made up of100 parts by weight of an ethylene-propylene-diene copolymer blendedwith 20 parts by weight of a polypropylene homopolymer having a meltflow rate of 10 g/10 min. (ASTM D1238 at 230° C.) and a density of 0.91g/cm³.

An ethylene-propylene-diene copolymer/polypropylene blend was cut intosamples having a width of 10 mm, a length of 60 mm and a thickness of 2mm. A 30-mm long area of each sample from its end was fusion bonded at200° C. to a 15-mm wide, 60-mm long flexible casing material from itsend.

An area of the sample that was not fusion bonded was then subjected totensile testing at a peel rate of 10 mm/min. by means of a tensiletesting machine (Strograph M100 made by Toyo Seiki Seisakusho) tomeasure peel strength. The results are reported in Table 1.

It is noted that a sample consisting only of theethylene-propylene-diene copolymer was peeled at the joining surface; inthe case of the ethylene-propylene-diene copolymer blended withpolypropylene, however, destruction occurred at the rubber site, and itis consequently found that strength at the joining surface was strongerthan actually measured. TABLE 1 Peel Strength (kN/m) Test No. EPDM aloneBlend of PP-EPDM 1 0.88 2.7 2 0.98 2.5 3 0.69 2.9 4 0.88 2.9 5 0.78 2.5Average 0.84 2.7

COMPARATIVE EXAMPLE 1

Thirty (30) electric double-layer capacitors were prepared, in which apressure release valve was prepared as in Example 1 with the exceptionof no provision of any liquid repellant porous film, and subjected tocharge-discharge cycle testing as in Example 5. Before the lapse of1,000 hours, the pressure release valves in four electric double-layercapacitors did not longer operate due to solid emissions precipitatedfrom the electrolytes, with a rupture of the flexible casing materials.

INDUSTRIAL APPLICABILITIES

The present invention successfully provides an electric double-layercapacitor sealed up by a flexible casing material, which ensures preciseoperation of a pressure release valve, and enables pressure to be safelyand reliably released out thereof upon an increased internal pressure.Thus, the electric double-layer capacitor of the invention has greatsafety and improved long-term reliability.

1. An electric double-layer capacitor sealed up by a flexible casingmaterial, characterized in that: a pressure release valve is attached tosaid flexible casing material, said pressure release valve comprises acollar air-tightly joined to the flexible casing material and acylindrical portion connected to the collar, extending out of acapacitor enclosure, and said cylindrical portion comprises an end parthaving a self-closing passage that is opening to the outside upon apressure release alone and is closed up in a normal state, and a channelin communication with an interior of the capacitor enclosure.
 2. Theelectric double-layer capacitor according to claim 1, characterized inthat said collar is provided thereon with a layer of material that iseasily joinable to an associated joining surface of the flexible casingmaterial.
 3. The electric double-layer capacitor according to claim 1 or2, characterized in that said end part having a self-closing passage isof a conical or truncated cone shape that diminishes in diameter towarda pointed end.
 4. The electric double-layer capacitor according to claim1 or 2, characterized in that said end part having a self-closingpassage is provided with a member that limits outward extension of saidend part.
 5. The electric double-layer capacitor according to claim 1 or2, characterized in that said end portion having a self-closing passageis formed of an olefinic synthetic rubber.
 6. The electric double-layercapacitor according to claim 5, characterized in that said olefinicsynthetic rubber is a polymer blend comprising a polypropylene and anethylene-propylene-diene copolymer.
 7. The electric double-layercapacitor according to claim 1 or 2, characterized in that said channelis provided with a porous member having a combined liquid repellency andgas transmission.
 8. The electric double-layer capacitor according toclaim 1 or 2, characterized in that the porous member comprises afluororesin.